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Borrowings - Additional information (Details)				3 Months Ended	9 Months Ended
	Apr. 06, 2021 USD ($)	Mar. 02, 2020 USD ($)	Feb. 24, 2020 USD ($)	Dec. 31, 2016	Sep. 30, 2021 USD ($) item	Dec. 31, 2020 USD ($)	Sep. 30, 2020	Apr. 20, 2018 USD ($)
Borrowings
Threshold percentage of stockholders' equity					10.00%
Borrowings at FHLBC as percentage of total assets					35.00%
Borrowings at FHLBC as percentage of book value of certain mortgage loans							60.00%
Bank owned FHLBC stock					$ 3,700,000	$ 3,700,000
Proceeds from term note		$ 20,000,000	$ 20,000,000.0
Cash distribution rate of trust preferred securities (as a percent)			7.80%
Senior notes					44,454,000	44,375,000
Basis points added to reference rate (as a percent)			175.00%
Total borrowings					192,534,000	160,521,000
GCFC/ABC Bank
Borrowings
FHLBC advance amount					$ 6,200,000
Debt assumed								$ 23,400,000
GCFC/ABC Bank | Maximum [Member]
Borrowings
Interest rate (as a percent)					2.83%
Three Months Secured Overnight Financing Rate
Borrowings
Basis points added to reference rate (as a percent)	273.00%
Securities sold under repurchase agreements
Borrowings
Carrying amount of securities secured					$ 43,000,000.0	67,000,000.0
Fair value of the pledged collateral					$ 113,800,000	94,400,000
Number of customers having secured balances exceeding specified percentage of stockholders equity | item					0
Total borrowings					$ 42,962,000	66,980,000
Federal Home Loan Bank Advances [Member]
Borrowings
Borrowings at FHLBC as percentage of book value of certain mortgage loans					60.00%
FHLBC advance amount					$ 0	0
Bank owned FHLBC stock					3,700,000
Principal balance of loans collateralized					553,600,000
Combined collateral value					402,100,000
Amount available for additional borrowings					269,700,000
Subordinated debentures
Borrowings
Total borrowings					59,190,000
Notes payable and other borrowings.
Borrowings
Total borrowings					20,155,000	23,393,000
Junior Subordinated Debt
Borrowings
Total borrowings					25,773,000	25,773,000
Senior notes
Borrowings
Interest rate (as a percent)				5.75%
Senior notes					44,500,000	44,400,000
Debt Instrument, Term				10 years
Interest rate term				5 years
Debt issuance costs					$ 546,000	625,000
Amortization period for deferred financing costs					10 years
Total borrowings					$ 44,454,000	$ 44,375,000
Senior notes | Debt Instrument, Variable Rate Base LIBOR [Member]
Borrowings
Basis points added to reference rate (as a percent)				385.00%
Term note
Borrowings
Proceeds from term note			$ 20,000,000.0		$ 14,000,000.0
Debt term			3 years
Subordinated Notes Due 2031
Borrowings
Interest rate (as a percent)	3.50%
Face amount	$ 60,000,000.0

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Represents the amount of borrowings from Federal Home Loan Bank as a percentage of book value of certain mortgage loans.

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Amount of long-term debt due within one year or within the normal operating cycle, if longer, assumed at the acquisition date.

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Percentage points added to the reference rate to compute the variable rate on the debt instrument.

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Face (par) amount of debt instrument at time of issuance.

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Contractual interest rate for funds borrowed, under the debt agreement.

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Period of time between issuance and maturity of debt instrument, in PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

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Amount of unused borrowing capacity under the long-term financing arrangement that is available to the entity as of the balance sheet date.

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Amount, after accumulated amortization, of debt issuance costs. Includes, but is not limited to, legal, accounting, underwriting, printing, and registration costs.

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Amount of the collateral pledged in support of federal home loan bank advances outstanding.

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Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

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